This invention relates to a so-called extrusion coating device.
In general, a so-called coated-type magnetic recording medium in which a magnetic layer is formed by coating a base film, such as a polyester film, with a magnetic paint composed of magnetic powders, binder, dispersant or lubricant dispersed and kneaded in an organic solvent, is used as an audio tape or a video tape.
As a method for coating the base film with the magnetic paint in such coated-type magnetic recording medium, a role coating method using a roll, exemplified by a gravure roll method or a reverse roll method, is mainly employed. However, these roll coating methods have some problems. For example, failure in paint transfer from the roll to the base film generates irregularities in coating thickness causing output fluctuations and splash of paint, which result in drop-out. Also, as excessively supplied paint is returned to the paint mixing process, the paint quality is deteriorated. These problems tend to be more conspicuous particularly in high-speed coating.
An extrusion (die) method has recently been noted and partly applied for practical use as a new coating method to solve the foregoing problems. The extrusion method employs a die 24 which has its distal end composed of an upper lip 21 and a lower lip 22 and has a wide slit 23 between the upper lip 21 and the lower lip 22 with the vicinity of the distal end of the slit 23 formed in a doctor-edged shape, as shown in FIG. 1. A paint 25 continuously extruded through the slit 23 is applied with a uniform coating thickness to the surface of a base film 26 running along distal end surfaces of the upper lip 21 and the lower lip 22.
Meanwhile, in the coating device for the extrusion method, shapes of distal end surfaces 27, 28 of the upper lip 21 and the lower lip 22 abutted against the base film 26 largely affect the product quality. Conventionally, the distal end surfaces 27 and 28 exhibit the following shapes. That is, the distal end surface 27 of the upper lip 21 is inclined in relation to the vertical plane orthogonal to the slit 23. The distal end surface 28 of the lower lip 22 is constituted by an inclined surface 28a formed by cutting out a portion of distal end surface in the vicinity of the slit in a wedge shape, and a curved portion 28b continuing from the inclined surface 28a and having a radius of curvature of 5-200 mm.
With such coating device, when the base film 26 is caused to run along the distal end surfaces of the upper lip 21 and the lower lip 22 with the paint being extruded through the slit 23 between the upper lip 21 and the lower lip 22, the paint extruded through the slit 23 is continuously applied on the base film surface with the inclined surface 28a of the lower lip 22. Therefore, this inclined surface 28a of the lower lip 22 greatly affects the coating characteristics. In consideration of the inclined surface 28a, particularly referred to as a chamfer, generating appropriate coating pressures, it is normal that the inclined surface 28a has an inclination angle of 5.degree.-60.degree. to the vertical plane orthogonal to the slit and a thickness of 0.5-2.0 mm. With respect to other portions, for example, the gap of the slit 23 is approximately 400 .mu.m, and the thickness of the lower lip is 3-5 mm.
However, in the coating device in which the distal end surface 28 of the lower lip 22 is composed of the chamfer 28a and the curved portion 28b continuing therefrom and having the radius of curvature of 5-200 mm, an edge of the chamfer 28a located on the boundary between the chamfer 28a and the curved portion 28b has an acute angle. Thus, following problems arise.
That is, the acute-angled edge of the chamfer 28a increases the possibility that the balance between the pressure of the paint and the tension of the base film changes on this edge. Thus, a minute change in the pressure of the paint or the tension of the base film alters the coating characteristics. Therefore, the coating speed cannot be raised higher than 500 m/min., which is disadvantageous to improvement of productivity.
Also, the acute-angled edge causes the distance between the chamfer 28a and the surface of the base film 26 to be relatively short. Consequently, the balance between the pressure of the paint and the tension of the base film tends to change easily in the width direction, and the change in the balance results in irregularities in coating thickness. Thus, it is difficult to reduce the thickness of the coated film.
In addition, the acute-angled edge of the chamfer 28a narrows the area of a portion supporting the base film 26 on the distal end surface of the die under the pressure of the paint. Therefore, the distance between the edge of the chamfer 28a and the base film 26 is reduced. When fine particles and dusts are present on the base film 26 in this case, the particles and dusts are trapped on the edge of the chamfer 28a, thus disturbing flow of the paint and causing coating defects, such as stripes.